1. Field of the Invention
The present invention is concerned with high toughness and high strength untempered steel having the mechanical properties equivalent to or better than those of tempered steel and processing method thereof, more particularly, the high toughness and high strength untempered steel having either the tensile strength higher than 75 kgf/mm.sup.2 with the impact toughness higher than 7 kgf-m/cm.sup.2 in the KS 3 specimen, or the tensile strength higher than 90 kgf/mm.sup.2 with the impact toughness higher than 5 kgf-m/cm.sup.2 in the KS 3 specimen, and processing method thereof.
2. Description of the Prior Art
Generally, the untempered steel means the steel which can exhibit the satisfactory mechanical properties in the work-hardened state without heat-treatments such as quenching-annealing and normalizing. However, since the toughness of untempered steel is extremely low compared to that of the tempered steel, its use has been limited to the crank shafts or other simple applications where the toughness is not considered as the important property.
Particularly, as pointed out in Japanese Patent Publication No.89-211606 or Japanese Patent Publication No. 83-53709, U.S. Pat. No. 4,851,054, since the conventional untempered steel exhibits its good mechanical properties only in the products with small diameter or thin plate but not in the products with large diameter, there have been many problems for the actual applications. Moreover, as indicated in U.S. Pat. No. 4,141,761, Japanese Patent Publication No. 79-66322, Japanese Patent Publication No. 83-167751 and Japanese Patent Publication No. 86-56235, the low-carbon and high-alloy steel has been developed, but there has been shortcoming that the induction heat-treatment cannot be performed on the steel.